The present invention relates to a safety switching device for connecting and reliably disconnecting an electrical load in response to a switching event of a safety transmitter. The invention particularly relates to a safety switching device having at least one output switching element and one operating mode selector unit for selecting an operating mode in response to a defined input signal.
The invention also relates to a method for selecting an operating mode of such a safety switching device.
Safety switching devices of the above-mentioned type are generally known. By way of example, the applicant offers various versions of safety switching devices under the name xe2x80x9cPNOZxe2x80x9d. Safety switching devices such as these are primarily used in the industrial area, in order to connect and reliably disconnect electrically driven machines, such as a press or a milling tool. They are used in particular in conjunction with a mechanically operable safety transmitter, for example an emergency off button, to disconnect the machine quickly and safely in an emergency situation. For this purpose, the power supply to the machine to be disconnected is passed via make contacts of two electromechanical switching elements. As soon as even only one of the two switching elements opens its make contacts, the power supply to the machine is interrupted.
The safety switching device thus carries out the task of safely evaluating the switching event produced by the safety transmitter and, in response thereto, of operating electronic and/or electromechanical switching elements, which then disconnect the power supply to the machine.
Since they are used in safety-critical areas, the safety switching devices referred to here require a specific operating approval, from the responsible supervisory authorities, in many countries. For this purpose, the intrinsic fail-safe nature of the devices must be verified in accordance with the Standards that exist, such as the European Standard EN 954-1. This has a considerable disadvantageous effect on the design freedom for the design and development of safety switching devices in comparison to xe2x80x9cnormalxe2x80x9d switching devices. In consequence, the expression xe2x80x9csafety switching devicexe2x80x9d is in this case intended to mean only those devices which have a relevant operating approval or, in the absence of such an approval, at least satisfy the requirements of Safety Category 3 of the above-mentioned Standard EN 954-1.
In addition to the emergency off switches which have been mentioned, a large number of other safety transmitters can be used, for example guard door switches, light barriers etc. Owing to the different requirements and alignments of these safety transmitters, the operating mode of the safety switching device needs to be matched to the type of safety transmitter that is used.
In this connection, the term xe2x80x9coperating modexe2x80x9d relates not only to the selection of specific types of operation of the device, for example whether (constant) steady-state or pulsed output signals are generated, but also to the selection of specific operating parameters, such as delay times of different duration for disconnection.
One possible way to match the devices to different requirements has simply been to provide a dedicated safety switching device for each type of safety transmitter. However, this involves increased storage requirements and problems with repair, since only that safety switching device which is respectively matched to the corresponding type of safety transmitter can be used.
In order to avoid this situation, the applicant has proposed a safety switching device as it is disclosed in DE 197 07 241 A1, where different safety transmitters from a set of possible safety transmitters can be connected. In this case, a typical combination of input ports and output ports to be used is assigned to each safety transmitter in the set. The way in which a specific safety transmitter is connected is unique for each safety transmitter within the set, by which means, on the basis of the connections made, the safety switching device can identify on start-up what type of safety transmitter is connected, and what function is required in the case of like safety transmitters. The safety switching device automatically selects the operating mode required on the basis of this association. In other words, the safety switching device has a plurality of input ports, and the user has the option to select one out of a number of operating modes by specifically connecting the safety transmitter to one of these input ports.
Although this safety switching device has been proven in practice, there is still a desire for improvement.
It is an object of the present invention to provide a safety switching device with high flexibility in terms of operating modes, but which is simpler in design. In particular, the safety switching device is intended to be capable of being produced at lower cost, while having a high flexibility.
It is another object to refine the safety switching device mentioned initially such that a number of operating modes can be selected by using a means with a simple design.
Just another object is to provide a safety switching device which capable of operating in one of a plurality of operating modes, with the selection of the operating mode desired being simple to achieve.
These and other objects are achieved by a safety switching device having an input, which is connected to the operating mode selector unit so that a selector input signal can be supplied to it, with the operating mode selector unit being designed such that it identifies the selector input signal as one of at least three different defined input signals, and, responsive thereto, selects one of at least three defined operating modes assigned to said different selector input signals. The operating mode selector unit can thus select one of at least three operating modes by evaluating a single signal at a single input.
In a method according to the invention the selector input signal is supplied to an operating mode selector unit via one input, with the operating mode selector unit identifying the selector input signal as one of at least three different predefined input signals, and selecting one of at least three predefined operating modes in response thereto.
The advantage of this safety switching device is, in particular, its simple design. It is sufficient to provide a single input for supplying the selector input signal. On the one hand, this allows the physical size of the safety switching device to be reduced while, on the other, it leads to cost savings.
A further major advantage is that there is no need for complex switching elements, which have to be designed in a redundant manner in order to achieve the required safety, for selecting the various operating modes.
In a refinement of the invention, the safety switching device comprises an internal signal generator, which generates the at least three different predefined selector input signals at one output at least.
This measure has the advantage that the at least three different defined selector input signals are always available, without any further effort. The safety switching device can thus be used without any additional generator means. Furthermore, it is particularly advantageous that, when a defective, old safety switching device is replaced, the new safety switching device is switched to the correct operating mode just by transferring the previous terminal wiring. This is very simple and convenient, and, furthermore, avoids safety-critical faults during the replacement of a defective device. In addition, this allows a defective device to be replaced more quickly than in the case of conventional selecting elements.
In a further refinement of the invention, the operating mode selector unit has a comparator for comparing the input signal with at least three different reference signals.
This measure leads to a simple design and, in particular, allows signals which are already present in the safety switching device, for example an operating voltage signal or a 0-volts signal etc., to be used as reference signals.
In a further refinement of the invention, a clock generator is provided, which generates a clock signal and provides this as one of the reference signals. The clock generator preferably generates two different clock signals, and provides these as two of the at least three different reference signals.
These measures make it possible to supply not only a signal at a constant voltage, that is to say for example at an operating voltage or at 0 volts, as the input signal, but to clock the input signal. This leads to the advantage that the input signal may assume different states, and thus also allows identification of different operating modes (more than two). From the design point of view, it is particularly advantageous to use four different input signals, namely a signal at an operating voltage, at 0 volts, with a first clock cycle or with a second clock cycle. Since the safety switching devices already have clock generator for two different clock signals in order to identify cross connections in connecting lines between a safety transmitter and the safety switching device, no additional complexity is therefore required in order to provide the reference signals. However, it is also feasible for a clock generator to be upgraded so that more than the said two clock signals are generated, thus allowing selection of more than four operating modes.
In a further refinement of the invention, at least one output port is provided, which is connected to the clock generator and provides one of the clock signals.
This measure has the advantage that a clock signal which has already been generated in the safety switching device for purposes of identifying cross connections can also be used for selecting the operating modes. It is therefore no need for any additional design measures in order to produce appropriate input signals for the operating mode selector unit.
In a further refinement of the invention, a control device is provided, which is connected to the operating mode selector unit and selects the desired operating mode in response to at least one output signal from the operating mode selector unit. The operating mode selector unit is preferably part of the control device.
It is furthermore preferred for the control device to generate an input signal for a downstream safety switching device, and to provide this at an output. This input signal may be a clock signal from the clock generator, a constant signal at a first voltage level, preferably ground, or a constant signal at a second voltage level, preferably an operating voltage.
These measures have the advantage that they make it possible for the control switching device to select the operating mode of the downstream, series-connected safety switching device. The downstream safety switching device can thus be matched to the type of output signal, for example a clock signal or a constant signal, from the upstream safety switching device.
In a further refinement of the invention, an input is provided for supplying a start signal, in order to select one out of a number of start operating modes. In this case, by way of example, possible start operating modes are xe2x80x9cautomatic startxe2x80x9d, xe2x80x9cmanual startxe2x80x9d or xe2x80x9cmanual monitored startxe2x80x9d.
This measure allows a further improvement in the flexibility of the safety switching device, since the desired start operating mode can be selected by choosing the start signal which is supplied via the start switch. The start signal, as in the case of the already explained input signal for selection of the operating mode, may thus be a clock signal or a constant signal.
In a further refinement of the invention, a selection gate is provided, to which the input signal and the start signal are supplied and which supplies one of the two signals to the operating mode selector unit in response to a control signal.
In other words, the already explained operating mode selector unit may also be used for selecting the start operating mode, with the control device sending an appropriate control signal to the selection gate, in order to select the start signal or the input signal. The particular advantage of this measure is design simplification, and hence cost savings.
Further advantages and refinements of the invention can be found in the description and the attached drawing.
It goes without saying that the features mentioned above and the features which are still to be explained in the following text may be used not only in the respectively stated combination, but also in other combinations or on their own, without departing from the scope of the present invention.